During the cement clinker calcination process, the kiln mouth rotates rapidly with the kiln body and is located in a position that is affected by rapid changes in temperature. This area requires special attention and maintenance during production.
The kiln mouth of the precalcining kiln operates under extreme conditions. The clinker exiting the kiln at a temperature of approximately 1350°C has the potential to cause abrasion and erosion to the castable material. The kiln rotates rapidly at a speed of about 4.0r/min, which causes the castable to be squeezed by the kiln guard iron and exposed to flames as high as 1700°C, resulting in significant heat radiation.
Additionally, the ellipticity of the kiln mouth shell can cause deformation and increase the likelihood of castable detachment. If the castable material at the kiln entrance fails during production, it must be capable of withstanding the rapid cooling and thermal shock caused by stopping and starting the kiln. In such cases, immediate shutdown of the kiln is necessary.
Several factors can affect the life of kiln portable castable
1. Including the negative pressure at the kiln head.
The negative pressure in the kiln is a result of the balance between the air used inside the kiln and the amount of hot air entering the kiln hood. When the exhaust air volume of the exhaust fan at the kiln head increases, the negative pressure at the kiln head also increases, and vice versa. Positive pressure at the kiln head can lead to various hazards, including flying sand material, hot flue gas, and damage to the kiln door and kiln head cover. In severe cases, it can even cause the kiln hood to catch fire.
High positive pressure can also damage peripheral equipment, limit visibility of the kiln head fire, and cause personal injury from splashed high-temperature materials. Therefore, it is essential to ensure that the exhaust fan at the kiln head has sufficient capacity to avoid remaining cooling air volume with nowhere to go.
In summary, the kiln mouth is a critical area that requires special attention and maintenance during cement clinker calcination. Understanding the factors that can affect the life of kiln portable castable, such as the negative pressure at the kiln head, is vital for safe and efficient production.
2. Kiln system operating rate
The position of the kiln mouth during the cement clinker calcination process is crucial as it rotates rapidly with the kiln body, and is susceptible to rapid temperature changes. It requires constant attention and maintenance during production.
The kiln mouth of the precalcining kiln is subjected to extreme temperatures of about 1350°C, which causes wear and tear on the castable. The rapid rotation of the kiln at a speed of about 4.0r/min, high-temperature expansion, and the intense heat radiation from the flame at the kiln head, which can be as high as about 1700°C, can cause the castable to get squeezed by the kiln guard iron.
The ellipticity of the kiln mouth shell is also deformed, increasing the possibility of the castable leaving the kiln body. When there’s an abnormality in the production of the kiln mouth castable, it must withstand the rapid cooling and thermal shock of starting and stopping the kiln. Generally, if the castable at the kiln mouth falls off, the kiln will be shut down immediately.
Factors Affecting the Life of Kiln Portable Castable
1. Negative pressure at the kiln head
The negative pressure at the kiln head reflects the balance between the air used in the kiln and the amount of hot air entering the kiln hood. When the exhaust air volume of the exhaust fan at the kiln head increases, the negative pressure at the kiln head increases, and vice versa. Positive pressure at the kiln head can cause flying sand material, hot flue gas, and damage to the kiln head cover, kiln door, and peripheral equipment. It can also cause personal injury in severe cases. During the design and matching of the system, the capacity of the exhaust fan at the kiln head should not be too small, as the remaining cooling air volume will have no place to go.
2. Kiln system operating rate
The comprehensive operating rate of the kiln system is an indicator of whether production is normal and management is fine and stable. If the comprehensive operating rate of the kiln system exceeds 90%, it is considered normal production, indicating few unplanned maintenance of the production line. The castable material at the kiln mouth is the most severely damaged during start and stop. The higher the comprehensive operation rate of the kiln, the longer the service life. In case of accidents, such as calciner shrinkage and blockage, the internal structure of the castable at the kiln mouth will be damaged, which will have a negative effect on the service life.
3. Kiln mouth guard iron structure
The kiln mouth structure is usually composed of a cylinder, back iron, and castable. The kiln mouth guard iron protects the kiln mouth shell section made of ordinary steel and avoids high-temperature flue gas burning. The back iron reduces the working temperature and abrasion and ensures the stability of the structure. The large vibration caused by the deformation of the kiln head cylinder and the looseness of the back iron bolts are the important reasons for the shedding of the castable at the kiln mouth.
4. The quality of the pouring material and the influence of the pouring construction quality
As mentioned earlier, ordinary castables cannot adapt to the harsh working conditions at the kiln mouth due to its particularity. The castables used should have high-temperature resistance, high strength, and good thermal shock performance. Currently, many manufacturers of kiln mouth castables in China offer them at a price ranging from 2,000 to 10,000 yuan. The service life of kiln mouth castables is generally proportional to the price.
There are several issues with construction quality control. The anchor nail’s structure is unreasonable, welding at the intersection is not standardized, and it is susceptible to burning at high temperatures. The anchor nails are single-angled, with few welding surfaces, and are difficult to weld firmly. The layout of the anchor nails is unscientific, leading to many blind spots, especially the guard plate and brick retaining ring, which the castable above cannot pull.
Due to insufficient understanding of the importance of leaving expansion joints during pouring, too little or no leaving in actual construction causes extrusion and bursting of kiln mouth castables. In an effort to save time during the construction period, the formwork was not supported, and the operating point was moved from 7 o’clock or 9 o’clock in the circumferential direction to 6 o’clock for construction, resulting in loose and non-dense castables.
The mixing process is not standardized, with water added before the dry material is fully mixed. Considering the convenience of construction and the influence of construction time, it is necessary to increase the amount of water for mixing and pouring with flowing material while stirring. The mixing and molding time do not match well, resulting in the stirred castable not being used on time. The pouring and rapping are uneven, and the phenomenon of long-term vibration or re-vibration is significant. Due to the urgency of production, too short curing time also causes severe damage to the castable.
5. The influence of the brick retaining circle at the kiln mouth
The service life of the castable at the kiln mouth and the integrity of the brick retaining ring at the kiln mouth are the most significant factors among many. The burning loss of the retaining brick ring at the kiln mouth forms an inverted triangle, and the kiln bricks will be directly squeezed onto the castable at the kiln mouth, causing a large area of the castable at the kiln mouth to be extruded and fall off, forcing the kiln to stop. Combined with the incidents of castable shedding of the kiln mouth protective iron that have occurred many times in our company in the past one and a half years, the kiln mouth retaining brick ring should be replaced as soon as possible once burned.
Solution:
1.Optimization of kiln mouth structure design:
The kiln mouth structure can be improved by changing the connection method of the back iron from a single block connection to a box type connection, which consists of an inner back iron and an outer box connected by countersunk bolts. The thickness of the pouring material for the straight wall below the kiln mouth should be increased to ensure that the end face of the kiln mouth is shrunk inside the straight wall without affecting the clinker falling into the grate cooler.
2.Strict quality control of castable construction and corresponding improvements:
1) To increase wear resistance, the effective thickness of the castable above the kiln mouth guard plate should be increased from 160mm to 220mm.
2) The anchor nails should be arranged rationally and materials with a relatively large welding area should be used for the anchor nail structure.
3) The principle of setting up formwork before construction should be strictly adopted when pouring the castable at the kiln mouth.
4) The castable should be stirred evenly by a special person, and expansion joints should be left every 600mm using 3mm plywood.
5) The castable material can be changed to a special castable for the kiln mouth to improve high temperature resistance and vibration resistance.
6) Priority should be given to the construction of the castable at the kiln mouth during maintenance, and the curing time of the castable should be prolonged as much as possible.
3. Improvement of kiln system ventilation to ensure slight negative pressure on kiln hood:
1) The slope ventilation at the kiln tail should be improved by adjusting the thickness of the original slope refractory material from 250mm to 200mm for AZM1680 silica brick masonry.
2) The air leakage of the kiln head system should be reduced as much as possible to improve the air volume handling capacity of the kiln head and meet the production requirements.
